Constant vs Switched Power - Car Stereo
 

I have a, hopefully, real quick question regarding the Constant wire and the Switched wire.

In my vehicle the Red is the Switched power coming from the Ignition.
The Yellow is the Constant from the battery, always 12V.

My question is this. After the truck is on. What wire does the head unit use as its main source of power? My thoughts are the Yellow wire, being constant from the battery. Using the Yellow just to say, "Hey, turn on!!"

Or is this "Stereo" specific?

The red wire is the main source of power, the yellow wire is constant from the battery thats used as power to retain the settings in your stereo when the vehicle is shut off such as your clock settings etc. Without this constant power you'd have to reset everything in your stereo every time you turn it on. This is not stereo specific, pretty much all stereos are set up this way.

Dang. Thanks though!

I'm wiring a relay to keep the stereo on after power has been removed, or cycled.

For instance, the normal start cycle:

• Insert key

• Turn key to ON position (warm the glow plugs) - Radio comes ON

• Rotate key to START (all power turns off to start engine) - Radio turns off

• Key returns to ON - Radio comes back on

Problem with this: Radio cycles power multiple times. Large DVD NAVIGATION stereos take along time to boot up. Plus, I don't feel that it is very good for a 1200 dollar head unit, to be power cycled during start up. Like pulling the plug on a PC.

Other problem: If waiting on the wife in the store that you just dropped her off at, and you want to turn the engine off, but leave the radio on, you must turn the key past OFF to put it in ACC.

A Time Delay Off-Relay will solve this problem. Just needed to know which wire is main power.

Will post pics, part numbers, and wiring schematic in a while!!

Thanks,

Always use the ACC power from the fuse block for switch power, this stops the cycling till its started.

On mine the Orange was switched and red was the source "power" Some are yellow, it just depends on the HU you have.

Yep, the red ones the one you want. You could try Muffinman's suggestion and see if that works, or something you could do that would be simpler than what you're talking about doing would be to install a jumper between the yellow and red with a switch on it. The only bad thing is that you'd have to remember to turn it on and off when needed. It'll be interesting to see what you come up with. Let me know if you have any other questions.

What kind of head unit do you have ? All the major aftermarket car stereo manufacturers have gone to standardized colors for their wiring which in the case of the power connections is red being switched, yellow being constant as shown in the following link. http://knowledge.sonicelectronix.com...standards.html I've never tried your suggestion about coming off the fuse block, I'll have to look into that. I would have thought that would loose power also when cycling through the different key positions. If this does work one would think the auto manufacturers would use a similar circuit to power the stock radios.

Never heard of the Red being the switch, All the ones I seen Red is the constant source. Then you can have either Yellow or Orange being the switched to turn on the power to raise the Antenna if it's electric, any amps, stereo or motors for fans. Some even have both orange and yellow for the signal wire.

Even the stock radio uses constant hot "Source" and an ACC "Signal" to turn the radio on after you start it or turn the key to the ACC position. this serves two purposes, Can't drain the battery if left on, and to stop the from damaging the system while cranking the engine over. The radio can't come on tell the ACC\Starter solenoid has closed.